Development of innovative cold-formed steel joist and composite floor systems

Widdowfield Small, Deighton (2023) Development of innovative cold-formed steel joist and composite floor systems. Doctoral thesis, Northumbria University.

Text (Doctoral thesis) widdowfield small.deighton_phd(15017488).pdf - Submitted Version Restricted to Repository staff only until 23 November 2025. Download (17MB) | Request a copy

Abstract

Environmental impact and the housing crisis are two major challenges faced by theconstruction industry. Reducing the industry’s impact on the environment while also increasing the rate of house building, to meet government targets, is only possible with the use of sustainable building materials, efficient design and modern methods of construction (such as modular construction).

Extensive numerical studies have been performed in this study on innovative cold-formed steel (CFS) and composite floor joists, validated against previously established experimental analysis. Comparison of failure mode, load-deflection curve and ultimate flexural capacity have shown that the finite element (FE) models created in this study are able to accurately predict flexural capacity. The flexural capacity of these innovative floor joists and systems have been studied using FE analysis software ABAQUS.

Innovative CFS floor joists have been designed to reduce instability due to buckling, which is critical failure mode of thin-walled members. To ensure pure bending failure of the CFS members four-point bending test set up has been used. Specifically, for CFS members studied in this research, which are the rectangular hollow flange beam (RHFB), built-up sigma section and sigma section with hollow flanges, parametric studies have been performed to assess the effects of yield strength, section thickness and section dimensions on the flexural capacity of innovative CFS floor joists.

Existing design rules, which tend to be conservative and therefore inefficient have been modified in this research for the RHFB member to better reflect the actual flexural behaviour of the section. Based on the direct strength method (DSM) modified design rules are presented to account for the inelastic reserve capacity. For the RHFB member increasing the section thickness had the greatest impact on flexural capacity.

Results of the parametric study conducted on the flexural capacity of the open built-up sigma section showed that section with 1 mm thickness failed largely by local buckling, whereas the 2 mm and 3 mm thick sections failed in distortional buckling. Shear failure was the primary mode of failure for section with a 10° bent angle and a depth greater than 240 mm. Increasing the bent angle to 60° led to a reduction in ultimate moment capacity.

The sigma section with hollow flanges studied in this research was found to be susceptible to local buckling and therefore current DSM design equations are therefore inadequate for use in predicting the flexural capacity of the section. Meanwhile in the parametric study performed increasing the section height led to the greatest increase in flexural capacity.

The innovative composite floor systems designed in this study substitutes carbon intensive materials, concrete, for timber. The floor system created by combining the cold-formed lipped channel beam and cross-laminated timber has been assessed in terms of flexural capacity. The proposed CLT-CFS composite floor system aims to reduce the material non-linearity and creep deformation of timber, thus improving the serviceability limit state (SLS) of the member. Combining the CLT panel with the lipped channel CFS beam was shown to have greater flexural capacity than the sum of the bare components by roughly 20%.

Additionally, a composite floor joist made by coming timber beams and glass fibre reinforced polymer (GFRP) has been studied. The mechanical properties of the GFRP pultruded reinforcement and the timber beams were validated separately and subsequently the flexural capacity of these beams was assessed. The GFRP added to the compression zone of the timber led to a positive influence on the ultimate moment capacity of the composite member.

In summary this research presents novel CFS floor joists and composite floor systems, studies their flexural capacity and in the case of the RHFB presents modified design equations to improve efficiency in design. This added efficiency means there can be a reduction in material use, carbon production and cost.

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